Human body communication means a technology for transmitting a signal between devices which are connected to a human body by using a human body having conductivity as a communication channel. That is, Human body communication means a technology for recovering transmitted information by transmitting information to an electrode of a transmitter attached to one part of a human body by using the human body having conductivity as a communication channel and contacting an electrode of a receiver to the outside of the human body attached to another part of the human body. Through the human body communication technology, communications between diverse portable devices such as Personal Digital Assistant (PDA), laptop computer, digital camera, MP3 player, and mobile phone, or communication with a fixed device such as communications between printers, payment for a credit card, TV reception, communication with an admission system, and payment for bus and metro fare are performed only by simple contact of a user.
A conventional human body communication technology realizes low-speed data communication as slow as tens of Kbps based on a specific frequency such as Frequency Shift Keying (FSK) or Amplitude Shift Keying (ASK). Application of the conventional human body communication technology is limited to a field requiring simple data transmission.
The conventional human body communication technology includes a technology using a photoelectric effect, which is a technology for applying a digital signal of Non Return to Zero (NRZ) directly to a human body and receiving the digital signal of NRZ based on the photoelectric effect. The conventional human body communication technology enables communication of 10 Mbps by remarkably improving transmission speed. The high-speed data transmission can be widely performed in a daily life by extending a limited application field.
In spite of improvement in the communication speed, the conventional technology using the photoelectric effect has a difficulty in adaptation to a small portable device due to a technical problem such as a size of a module, consumption power, and interference by another lighting.
As a method for solving the above problem, technologies using an electronic recovering method are introduced. Since these technologies realize a single chip as well as communication of 2 Mbps, it shows that the technologies are adaptable to diverse electronic devices in daily life.
The technologies using an electronic recovering method may be an optimized technology for realizing a network between diverse sensors adaptable to the human body such as ElectroCardioGraph (ECG), Non-Invasive Blood Pressure (NIBP), and Heart Monitor, and a human body-based network such as a wearable computer, which has not been easily realized.
Since the human body is formed of diverse materials and has diverse formats, low conductivity and high dielectric constant, the human body functions as an antenna in a wide frequency region.
Although it is possible to communicate by using the human body as the antenna due to the above characteristics, a signal of another neighboring user or un-desired noise signals from external electronic devices may be transmitted to the human body. Also, there is a possibility that communication becomes unstable due to the distance between peripheral objects or devices, or the location of the peripheral objects or devices.
A frequency modulating method such as FSK, ASK, and Phase Shift Keying (PSK) selectively uses a region of low external interference and requires a low signal-to-noise rate in realization of communication. However, when the interference occurs, it is difficult to solve the problem.
A method for directly transmitting a digital signal requires a wide band. Also, when characteristics of a reception signal are recovered, strong interferences transmitted from peripheral devices except the signal transmitted from the transmitter are also inputted. When interference occurs within a signal band, a desired signal may not be well divided with no regard to sensitivity of the receiver. A bandwidth larger than tens of MHz is required to directly a digital signal larger than Mbps. When these signals are authorized to the human body, a signal larger than a specific frequency is radiated. Accordingly, although a plurality of users do not contact each other, forming a stable network becomes difficult by generating interference to other users.
Therefore, a method for limiting an occupied frequency of a signal transmitted through the human body within a range of frequency which does not affect neighboring people is suggested for a stable communication among a plurality of neighboring users without interference. When the human body is used as a channel and a usable frequency is limited, a communication speed is remarkably limited. At present, the maximum communication speed realized according to the method is 10 Mbps. In the method, since a transmission signal includes many high-frequency signals, signals of tens of MHz are applied to the human body in consideration of an occupied frequency band. Since these components do not stay in the human body, but are radiated, interference occurs over a neighboring user. That is, since interference is generated between users when a plurality of users exist in a limited space, it is difficult to realize the stable communication in the conventional technology.